Conventionally, in a case where a reception signal is weak, an amplifying circuit for amplifying the signal is used in a communication device and the like. However, in the amplifying circuit, not only the reception signal but also noise is amplified. Therefore, it is necessary to provide a signal detector that determines whether the signal amplified by the amplifying circuit is the reception signal or noise (that is, presence or absence of the reception signal). In a case of the reception signal, a signal processing circuit at the post stage of the signal detector performs signal processing on the reception signal. In a case of the noise, the signal processing is not performed. For example, such a signal detector is proposed that a comparator that compares the signal level of the amplified signal with a fixed reference level is provided and the presence or absence of the reception signal is detected on the basis of whether or not the signal level is higher than the reference level (e.g., refer to Non-Patent Literature 1).